I've been on either end of it. More on the being bullied, but I have done some bullying before, not that that's how I thought about it at the time. It's pretty shameful, but I can also see why I did it. Mostly it was to fit in with people (even though they were people I didn't really like - how's that for dumb?)
I have, and MANY others I can imagine, have teased or excluded someone in order to 'fit in'. Like you said, I did this on occasion to be 'in' with people I didn't really care for mainly because of these pressures. As I get older, I like to hope I am reducing some of this inadvertent bullying, especially exclusion, which is often overlooked. I am one of those people that feels extreme guilt about a couple of seemingly minor incidences of bullying, for example, not letting someone sign my year book, claiming, " the blank pages were all full". Certainly I can justify that I didn't really know this girl well, she may have developed a crush on me since I was one of the few people who was nice to her, but...man, it's so petty to risk someone else's feelings over something as minute as a yearbook note.
And, of course, like anyone, I have also endure many forms of bullying. And, thinking how I felt, in makes me feel even more ashamed for any behaviors I had engaged in. Though, despite the apparent subjectivity, I would guess I have bullied others less than acts of bullying against me. With that said, overtime, any form of exclusion or rude remarks slung at me really doesn't get to me much at all anymore. Moreover, with more confidence and immunity to these sorts of exchanges, the frequency of them are very low as well. I have just tried to be independent and amiable to others, and being positive, successful, and confident during the post-high school years has almost completely taken the target off me.